How Is the Switching Capacity of a Relay Defined and What Determines It?

Issue Details

One of the specifications on switches and relays is the switching capacity. What does this mean and how is it determined?

Solution

Switching capacity or switching load is usually rated with both voltage and current. The voltage is the load voltage which will be across the terminals of the relay when it is open. The current is the load current that will flow through both the load and relay when it is closed.

Switches often specify a minimum switching capacity (sometimes referred to as minimum switch load) in addition to a maximum switching capacity.  It is important to meet the minimum capacity of a switch to ensure long term health of the switch.  Over time particulates build up on the contacts of armature relays.  A minimum amount of current is required to burn off particulate build up on switch contacts when the relay is closed.  This is only a concern for electromechanical armature relays.  Reed relays are sealed in a noble gas and are not susceptible to particulate buildup.  SSR and FET relays are not mechanical and are also not affected.  
 

Additional Information

Relays theoretically do not dissipate any power. The equation for power is Power=Voltage*Current. When the relay is open, the current is zero. When it is closed, the voltage across the terminals is almost zero (depending on the load current and "on" resistance of the relay). In both cases, power is equal or very close to zero. Ideally when the relay opens, the voltage will instantaneously go to the load voltage and the current will instantaneously go to zero. If this were true, then the switching capacity would be as large as the gauge of wire used in the relay.

However, when the relay is in the process of switching, there is a finite time when both values are nonzero values. During this time, power will be dissipated in the relay. Depending on the inductance and capacitance of the load and relays, there may be large voltage and current spikes during this period of time when the relay is switching. This may generate a lot of heat in a very small area and consequently melt the contacts of the relays or leave corrosion that will shorten their life. With larger voltages and currents on your load, the switching time will be longer, and you will increase the energy (the time integral of power) dissipated in your relay. You may also have larger power spikes as the relay is in transition.

Another factor of the switching capacity has to do with the mechanical construction of the relay. When the relay is just beginning to switch, the force of the contact decreases which results in an increase of resistance. The load current flowing through this resistance will heat the contacts. There is also an instant when some parts of the contact are touching and some are not. By decreasing the surface area that the current is flowing through, the resistance is again increased. At the instant just before there is no contact, all of the current is flowing through an extremely small area. This last point of contact will get so hot that it will usually boil or even vaporize.

In practice, this issue often comes up with usage of the device in unexpected scenarios. For example, a PXIe-2532B in a 4 by 256 matrix will not be able to do a Fault Insertion Unit (FIU) Test because it's limit is sixty drives continuously. This is a direct example of the above mentioned theory and can be resolved by using a specific FIU scheme or be using an alternative PXI-2536 FET that can take multiple drives.